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Bi underpotentially deposited on Au(111) has been studied using potential-step chronocoulometry to determine the actual surface coverage of Bi. In the potential region where this system exhibits catalytic activity for the electroreduction of peroxide to water, the observed coverage is 0.25 monolayer (ML), which agrees well with the coverage of the reported (2 × 2) Bi overlayer observed by scanning probe microscopy in this region. At more cathodic potentials, the coverage increases to 0.67 ML. This coverage agrees with that expected based on the (p × √3) structure proposed from scanning tunneling microscopy, atomic force microscopy, and SXS measurements in this region. The electrosorption valency calculated based on these coverages is 3, indicating that the Bi is fully discharged on the surface. Potential-step chronocoulometry has been used at various pH values to determine the surface coverage of hydroxide anion in the presence of underpotentially deposited (upd) Bi. The coverage is negligible in the absence of upd Bi and at potentials where the Bi adlayer condenses. It rises to a peak of 0.17 ML in the region where the coverage is 0.25 ML, indicating that OH- is coadsorbed with the Bi.

Cd underpotentially deposited on Au(111) has been studied using potential-step chronocoulometry and electrochemical quartz crystal microbalance investigations to determine the actual surface coverage of Cd. In the potential region where Cd undergoes underpotential deposition (upd), the coverage is observed by chronocoulometry to rise to a limiting value of 0.66 ML. The electrosorption valency calculated on the basis of this coverage is 0.5, indicating that the Cd adatoms are not fully discharged on the surface. Potentialstep chronocoulometry has been used at various sulfate concentrations to determine the surface coverage of sulfate anion in the presence of underpotentially deposited Cd. The coverage is negligible in the absence of underpotentially deposited Cd at the point of zero charge. It rises to a value of 0.23 ML in the Cd upd region, indicating that sulfate is specifically adsorbed with underpotentially deposited Cd at negative electrode potentials. Quartz crystal microbalance measurements indicate that there is considerably larger mass change at the surface during Cd upd than that expected on the basis of the measured coverages of Cd and sulfate. The extra mass is explained in terms of additional sulfate required in the outer Helmholtz plane to balance the residual positive charge on the underpotentially deposited Cd atoms.

ABSTRACTα-Dodecatungstosilicate (α-SiW12O404−) anions form ordered monolayers on Ag(111) and Au(111) surfaces. In-situ STM images reveal that the silicotungstate ion forms a square adlattice with an intermolecule spacing of 10.2 ± 0.5 Å on both Ag and Au surfaces. Additional structures exhibiting either row or rhombic motifs are observed on Au electrodes. The structure of the adlattices can be modeled using a simple model which maximizes the coordination of the silicotungstate ion to the electrode while maintaining van der Waals contacts between terminal oxygens of adjacent silicotungstates.

Tl underpotentially deposited (upd) on Au(111) has been studied using potential-step chronocoulometry and electrochemical quartz crystal microbalance to determine the actual coverage of Tl. At anodic potentials, Tl adsorbs to the surface at a coverage that slowly increases up to 0.55 ML. At more cathodic potentials, the coverage rises sharply to a plateau of 0.79 ML where the surface is covered with a close-packed Tl monolayer. The electrosorption valency calculated based on this coverage data indicates that the low coverage overlayer is incompletely discharged but that the overlayer undergoes further discharge during the voltammetric features associated with complete monolayer formation. Voltammetric experiments also indicate that the presence of hydroxide anions plays a role in stabilizing the charged low coverage phase. The implications of this structural and charge information for the catalytic activity of the Tl upd system are discussed.